Stitching by inserting curable compliant materials of parts produced via additive manufacturing techniques for improved mechanical properties

ABSTRACT

The invention provides a method for the production of a 3D printed object ( 100 ), wherein the method comprises (i) a 3D printing stage, the 3D printing stage comprising 3D printing a 3D printable material ( 110 ) to provide the 3D printed object ( 100 ) of printed material ( 120 ), wherein the 3D printing stage further comprises forming during 3D printing a channel ( 200 ) in the 3D printed object ( 100 ) under construction, wherein the method further comprises (ii) a filling stage comprising filling the channel ( 200 ) with a curable material ( 140 ) and curing the curable material ( 140 ) to provide the channel ( 200 ) with cured material ( 150 ), wherein the cured material ( 150 ) has a lower stiffness than the surrounding printed material ( 120 ).

FIELD OF THE INVENTION

The invention relates to a method for the production of a 3D printedobject. The invention also relates to such object per se, for instanceobtainable with such method.

BACKGROUND OF THE INVENTION

Additive technologies wherein a material is incorporated in an objectmade via such technology are known in the art. US2013303002, forinstance describes a three-dimensional interconnect structure formicro-electronic devices and a method for producing such an interconnectstructure. The method comprises a step wherein a backbone structure ismanufactured using an additive layer-wise manufacturing process. Thebackbone structure comprises a three-dimensional cladding skeleton and asupport structure. The cladding skeleton comprises layered freeformskeleton parts that will form the electric interconnections between theelectric contacts of the interconnect structure after a conductivematerial is applied on the backbone structure. The support structuresupports the layered freeform skeleton parts. Parts of the supportstructure may be removed to isolate and/or expose the electricinterconnections. The cladding skeleton can be embedded by an insulatingmaterial for providing a further support. Amongst others, the claddingskeleton parts form a single connected tube that is cladded on an insidesurface by flushing a plating fluid trough the tube for forming theelectric interconnections.

Additive manufacturing (AM) is a growing field of materials processing.It can be used for rapid prototyping, customization, late stageconfiguration, or making small series in production. For 3D inkjetprinting or dispensing, a liquid monomer mixture may be used and byphoto polymerization the shape is then fixed before the following layeris deposited.

U.S. Pat. No. 5,700,406 discloses an method for manufacturing an articlewherein a cavity if formed by jetting droplets of liquid build materialonto a platform, and wherein the cavity is filled with a curablematerial to form a solid portion of the article

Most of AM technologies produce objects with mechanical propertiesinferior to that of objects produced by conventional manufacturingprocesses. For instance, in-plane and out-of-plane mechanical propertiesof AM structures may be inferior to injection molding (IM)structures/materials due to high anisotropic behavior of these objects.Further, low in-plane properties are a result of the heterogeneity ofthe structure. For example, in Fused Deposition Modelling (FDM), theheterogeneity is caused by the shape of the fibers and the voids inbetween the fibers. This induces e.g. localization of deformation in thefiber, possibly leading to fiber fracture, and possible detachment offibers. Yet, low out-of-plane properties are a result of theheterogeneity of the structure and of the process: due to the timedifference (and thus temperature difference) in the layer-by-layerprocessing, the adhesion between layers is lower than the adhesionbetween fibers within the layers. Hence, not only the deformation islocalized in the fiber and the fibers detach, but also the layers(partly) detach. Especially the inelastic properties suffer from theseoccurring phenomena (as we are talking about fiber fracture anddetachment which do not occur in the elastic regime).

SUMMARY OF THE INVENTION

Hence, it is an aspect of the invention to provide an alternative methodfor 3D printing, which preferably further at least partly obviates oneor more of above-described drawbacks. It is also an aspect of theinvention to provide an alternative 3D printed object, which preferablyfurther at least partly obviates one or more of above-describeddrawbacks.

Herein, we propose to introduce e.g. stitches, or other types ofreinforcing structures, in AM produced objects to improve theirmechanical properties. The stitches, or other types of reinforcingstructures, are made of material with a high compliance (i.e. lowstiffness) and ductility (i.e. large strain to break). This may resultin e.g. (a) lower stresses (due to compliance of the inserted material)in the fibers thereby preventing, or postponing, localization ofdeformation and fracture of the fibers, (ii) improved out-of-planemechanical integrity of the material, and (iii) that the insertedmaterial functions as crack stopper, due to its high ductility. Thestitches, etc., may be produced by filling cavities in the printedobjects with a low viscosity material that can be cured at a laterstage. Such cavities may be deliberately induced in the 3D object duringprinting.

One could envision including a “conventional” stitching process in theadditive manufacturing machine, however the “conventional” stitches addto the thickness of the part. For clothes, for example, this does notmatter as the material itself as well as the stitching thread is softand flexible, so that in the final product no unevenness due to thestitches is felt. For other, harder and stiff materials this mighthowever be a problem. Also, stitching in a “conventional way” might becomplicated as it requires making holes in the manufactured part. Toavoid those problems, with 3D design and printing the stitches, or othertypes of reinforcing structures, can be introduced inside the materialas shown below also with the help of sacrificial layers as discussedfurther below.

In a first aspect, the invention provides a method for the production ofa 3D printed object (“object” or “3D object”), wherein the methodcomprises (i) a 3D printing stage wherein a 3D printable material(“printable material”) is 3D printed to provide the 3D printed object,and wherein during 3D printing a channel is formed in the 3D printedobject (which is under construction), the channel comprising two or morechannel parts, each channel part having a channel axis, and two or morechannel axes having a mutual angle larger than 0° and smaller than 180°,and wherein the method further comprises a filling stage wherein thechannel is filled with a curable material and wherein the curablematerial is cured to provide the channel with cured material, the curedmaterial having a lower stiffness than the surrounding printed material.The channel may thereby provide a reinforcing structure, substantiallyembedded in the printed 3D object.

With such method, a 3D printed object can be obtained that has a higherstrength. Delamination, for instance, may be inhibited by the channelfilled with cured material. Such channel may be used and configured as akind of stitch and/or anchor. A further advantage of the invention isthat the channels, which may (thus) also be indicated as reinforcingchannels, may be configured in such a way, that they are not visiblefrom the outside. This may add to the appearance of the object and mayalso provide more smooth surfaces.

The terms “3D printed object” or “3D object” refer to a threedimensional object obtained via 3D printing (which is an additivemanufacturing process), such as an object having a height, a width and alength. The 3D object can in principle be any object that is 3Dprintable. It can be an item with a use function or a purely decorativeitem. It can be a scale model of an item such as a car, a house, abuilding, etc. Further, the 3D object can be a piece or element for usein another device or apparatus, such as a lens, a mirror, a reflector, awindow, a collimator, a waveguide, a color converting element (i.e.comprising a luminescent material), a cooling element, a lockingelement, an electrically conducting element, a casing, a mechanicalsupport element, a sensing element, etc. The 3D printed object comprises3D printed material.

Additive Manufacturing (AM) is a group of processes makingthree-dimensional objects from a 3D model or other electronic datasource primarily through additive processes. The additive process caninvolve the binding of grains (via sintering, melting, or gluing) or oflayers of material (via successive deposition or production of thelayers, e.g. polymerization). In the case of selective melting of metalgrains, it is possible to produce objects with similar density andstructure as objects produced by conventional manufacturing processes.In the other cases, the objects are composed of grains or layersseparated by interfaces that will influence the properties of theobject.

A widely used additive manufacturing technology is the process known asFused Deposition Modeling (FDM). Fused deposition modeling (FDM) is anadditive manufacturing technology commonly used for modeling,prototyping, and production applications. FDM works on an “additive”principle by laying down material in layers; a plastic filament or metalwire is unwound from a coil and supplies material to produce a part.Possibly, (for thermoplastics for example) the filament is melted andextruded before being laid down. FDM is a rapid prototyping technology.Another term for FDM is “fused filament fabrication” (FFF). Herein, theterm “filament 3D printing” (FDP) is applied, which is considered to beequivalent to FDM or FFF. In general, FDM printers use a thermoplasticfilament, which is heated to its melting point and then extruded, layerby layer, (or in fact filament after filament) to create a threedimensional object. FDM printers can be used for printing complicatedobject. Hence, in an embodiment the method includes production of the 3Dprinted object via an FDM 3D printing.

Materials that may especially qualify as 3D printable materials may beselected from the group consisting of metals, glasses, thermoplasticpolymers, silicones, etc. Especially, the 3D printable materialcomprises a (thermoplastic) polymer selected from the group consistingof ABS (acrylonitrile butadiene styrene), Nylon (or polyamide), Acetate(or cellulose), PLA (poly lactic acid), terephthalate (such as PETpolyethylene terephthalate), Acrylic (polymethylacrylate, Perspex,polymethylmethacrylate, PMMA), Polypropylene (or polypropene),Polystyrene (PS), PE (such as expanded-high impact-Polythene (orpolyethene), Low density (LDPE) High density (HDPE)), PVC (polyvinylchloride) Polychloroethene, etc. Optionally, the 3D printable materialcomprises a 3D printable material selected from the group consisting ofUrea formaldehyde, Polyester resin, Epoxy resin, Melamine formaldehyde,Polycarbonate (PC), rubber, etc. Optionally, the 3D printable materialcomprises a 3D printable material selected from the group consisting ofa polysulfone, a polyether sulfone, a polyphenyl sulfone, an imide (suchas a poly ether imide) etc.

The 3D printed object is especially (at least partly) made from 3Dprintable material (i.e. material that may be used for 3D printing). Theterm “3D printable material” may also refer to a combination of two ormore materials. In general these (polymeric) materials have a glasstransition temperature T_(g) and/or a melting temperature T_(m). The 3Dprintable material will be heated by the 3D printer before it leaves thenozzle to a temperature of at least the glass transition temperature,and in general at least the melting temperature. Hence, in an embodimentthe 3D printable material comprises a thermoplastic polymer, such ashaving a glass transition temperature (T_(g)) and/or a melting point(T_(m)), and the printer head action comprises heating the one or moreof the receiver item and 3D printable material deposited on the receiveritem to a temperature of at least the glass transition temperature,especially to a temperature of at least the melting point. In yetanother embodiment, the 3D printable material comprises a(thermoplastic) polymer having a melting point (T_(m)), and the printerhead action comprises heating the one or more of the receiver item and3D printable material deposited on the receiver item to a temperature ofat least the melting point. Specific examples of materials that can beused (herein) can e.g. be selected from the group consisting ofacrylonitrile butadiene styrene (ABS), polylactic acid (PLA),polycarbonate (PC), polyamide (PA), polystyrene (PS), lignin, rubber,etc. Other examples of materials that can be used can e.g. be selectedfrom the group consisting of metal, clay, concrete, etc.

The 3D printing technique used herein is not limited to FDM. Other 3Dprinting techniques that may also be applied in the invention may e.g.be selected from stereo lithography, powder binding, ink-jetting, etc.An extrusion based process such as FDM may be applied, wherein amongstothers one or more of thermoplastics (e.g. PLA, ABS), HDPE, eutecticmetals, rubber, silicone, porcelain, etc. may be applied. Anotherextrusion based process, such as robocasting may also be applied,wherein amongst others one or more of a ceramic material, a metal alloy,a cermet, a metal matrix composite, a ceramic matrix composite, etc. maybe applied. Electron beam freeform fabrication (EBF3) or direct metallaser sintering (DMLS) may be applied, wherein especially a metal alloymay be applied. Further, (granular-based) processes such aselectron-beam melting (EBM) (especially with a metal alloy), selectivelaser melting (SLM) (especially with a metal or a metal alloy),selective laser sintering (SLS) (especially with a thermoplasticpowder), etc., may be applied. Further, powder bed and inkjet head 3Dprinting may be applied, such as plaster-based 3D printing (PP), withe.g. plaster. Also laminated object manufacturing (LOM), with e.g.paper, metal foil or plastic film, may be applied. Further stereolithography (SLA) or digital light processing (DLP), with e.g. aphotopolymer, may be applied, etc.

Especially, however, the method includes printing of polymers, i.e. theprintable material and the curable material may comprise polymericmaterial, especially differing from each other.

In a specific embodiment the curable material may comprise one or moreof a polysiloxane, a polysilazane, a polyurethane, an epoxy, apolyamide, a polyimide, a polyester, and an acrylate; especially asilicone-type polymer, such as PDMS. In yet a further embodiment (seealso above), the 3D printable material may comprises one or more of apolymeric material selected from the group consisting of ABS,polystyrene and polycarbonate (PC). The printable material may alsocomprise other materials (see also above and below). The printablematerial may be solid at room temperature, but upon heating may becomeprintable (i.e. especially flowable). Downstream from a printer nozzle,optionally further heating may be applied, for instance to cure theprintable material.

The terms “upstream” and “downstream” relate to an arrangement of itemsor features relative to the propagation of the printable material from aprintable material generating means (here especially the nozzle of the3D printer (head)), wherein relative to a first position within a flowof the printable material from the printable material generating means,a second position in the flow of the printable material closer to theprintable material generating means is “upstream”, and a third positionwithin the flow of printable material further away from the printablematerial generating means is “downstream”.

In general, the channel will not be a straight channel, without anycurvatures relative to a channel axis. The channel will in general havea plurality of curvatures and/or other means to improve integrity of the3D object. Of course, the 3D object may include a plurality of channels.The term “channel” may thus also refer to a plurality of channels.

The channel comprises two or more channel parts forming said channel,wherein each channel part has a channel axis, and wherein two or morechannel axes have a mutual angle (α) larger than 0° and smaller than180°. The channel may also comprise at least three channel parts havingat least three channel axes having mutual angles (α) larger than 0° andsmaller than 180°. By including curvatures, one may further support thatdifferent parts of the 3D object may not easily break; the channels,more precisely the cured material inside the channels may keep the partstogether. Especially, the channel may be configured as meanderingstructure in the 3D object. An example thereof may be a stitch, such asa chain stitch. Especially, the channel may comprise two channel partshaving a channel (part) axis having an acute angle; even more especiallythe channel may comprise three or more channel parts having channel(part) axes having acute angle. As indicated above, such channels maynot be visible from the outside of the object.

The channels are especially substantially entirely filled. Further, thecurable material is especially not applied as a kind of film to thechannel walls, but substantially the entire cross-section of the channelmay be filled with curable/cured material. The channels may have acircular cross-section but the channels may also have other type ofcross-sections, like square, triangular, etc. Further, the cross-sectionover the length of the channel may vary. For instance, when using FDM,the channel may also include e.g. voids generated during FDM printing.

Also different types of channels may be applied. In general theequivalent circular diameter (2*sqrt(Area/π) wherein “sqrt” is thesquare root) will be in the range of 0.05-100 mm, such as 0.2-50 mm,which may depend upon the size of the 3D object. As indicated above,even when complying with this equivalent circular diameter, the shape ofthe cross-section may vary over the channel length. In general, thetotal channel volume of the channels filled with cured material relativeto the total volume of the printed material including the channelsfilled with cured material may be in the range of 0.05-20 vol. %, suchas 0.5-10 vol. %. Further, in general the channel(s) will be filled withcured material in the range of at least 70 vol. %, such as at least 80vol. %, even more especially at least 90 vol. % of the channel volume,such as substantially entirely filled with cured material.

The objects may also include different layers, such as layers comprisingdifferent chemical compositions. Such layered structure might include aninherent weakness, as the layers may tend to delaminate, e.g. undercertain types of stress. Hence, in an embodiment the 3D printed objectcomprises two or more layers, wherein the channel is configured withinat least part of a first layer and at least part of a second layer.These first and second layers may not necessarily be adjacent.Optionally one or more layers may be configured in between. In yetanother embodiment, the layers are in physical contact with each other.

For further increasing reinforcement, the channel may include structuresor elements which anchoring a body part to another body part. Herein,the term “body part” does not necessarily refer to a part having adistinguished function from another body part. The “term body” part mayespecially refer to a part of a body that may optionally besubstantially identical to one or more other body parts. Suitablestructures are e.g. anchor structures. Hence, in a further embodimentthe channel comprises an anchoring part selected from the groupconsisting of a bifurcation structure and a stitch structure. With abifurcation structure, the channel may split (“pure bifurcation”) in twoor three (“crossing”) or more channels. A stitch structure is a kind ofloop structure, optionally including knots, etc. The stitch structuremay optionally be a repeating stitch structure. Hence, especially eachchannel may comprise one or more anchoring parts.

As indicated above, when providing the channel, especially includinganchoring parts, different body parts (even while being identical interms of composition, and even while not being distinguishable from eachother), may be kept together better than without such structures.Breaking or delaminating may be prevented. However, as also indicatedabove, the anchoring parts may of course also be used for body partsthat have different compositions or textures, etc. Hence, in a furtherembodiment a first anchoring part is configured in a first layer andwherein a second anchoring part is configured in a second layer, whereinthe first and the second layer especially have different chemicalcompositions.

The channels are provided during 3D printing. The 3D printing leads to a3D object. The product obtained between t=0 sec (the start) and the lastprinting action is herein also indicated as 3D object. However,sometimes this object, when it is being made, is also indicated as 3Dobject under construction. For instance, this nomenclature can be usedto stress that the action is executed during the 3D printing process.Therefore, the 3D printing stage (further) comprises forming during 3Dprinting a channel in the 3D printed object under construction. Hence,in fact the channel is printed, i.e. the 3D object is printed in such away that a channel is formed during 3D printing.

The filling of the channels may be done during printing. For instance,part of a channel is formed, or a channel is ready, and then the channelis filled with curable material, followed by an optional curing, andfurther 3D printing (optionally followed by curing; of course at leastone curing stage is applied to cure the curable material), which further3D printing may optionally also further include the generation ofchannels and filling of the channels with curable material. In yetanother embodiment, however, first the object is substantially entirely3D printed, followed by the filling of the channel(s). Hence, the methodmay thus further comprise (ii) a filling stage comprising filling thechannel with a curable material (and curing the curable material toprovide the channel with cured material).

Nevertheless, after the curing a final (3D printing) action may beexecuted, for instance to provide a closure layer to close an opening ofthe channel. Hence, in an embodiment the method further comprises (iii)a finishing stage subsequent to the filling stage, wherein the finishingstage comprises closing a channel opening, optionally also by 3Dprinting. Note that this finishing stage, or more precisely, the closingof the channel, is not always necessary. For instance, one may acceptthe fact that a cured material is visible at the end of a channel at anouter surface of the 3D object. Note that the finishing stage mayoptionally also include one or more of (a) heating (such as by a laserand/or a flame) at least part of the outer layer of the 3D object, (b)solvent dissolving at least part of the outer layer of the 3D object,and (c) coating at least part of the outer layer of the 3D object.Alternatively, the finishing stage may be subsequent to the filing, butbefore the curing. Hence, optionally curing is only done after the 3Dprinted object is entirely printed. Hence, optionally the filing stageand finishing stage may at least partly overlap.

The channels may be filled with a liquid (curable material), forinstance by injecting a curable liquid, such as with a syringe. Thiscurable material may especially have a relatively low (dynamic)viscosity, such as 100-1000 cP (at 20° C.) prior to curing. Additionallyor alternatively, a vacuum may assist the filling. Hence, in a furtherembodiment the filling stage comprises subjecting the 3D printed objectto sub-atmospheric pressure and subsequently filling the channel withthe curable material. Alternatively or additionally, an apertureconnected to the channels to be filled in the 3D printed object (underconstruction) can be used as a vacuum inlet.

The filling material may especially also have a low shrinkage uponcuring (typically smaller than several volume %) and the coefficient ofthermal expansion should especially be close to the 3D printed materialin the range of possible operating temperatures of the printed device toreduce processing-induced residual stresses. Hence, especially a ratioof the thermal expansion of the printed material and of the curedmaterial may especially be in the range of 0.6-1.4, like 0.7-1.3, suchas 0.8-1.2, like 0.9-1.1. The cavities are easily introduced in theobjects during the printing process by making use of the provided designfreedom in additive manufacturing techniques that allow the realizationof complex 3D shapes within the actual products, e.g. cork-screw orplant root like.

As indicated above, the filling material is a curable material. Curingmay for instance be executed by one or more of light and heat, as knownin the art. Would the 3D object include a radiation transmissivematerial, such as a material transmissive for one or more of UV, visibleand IR radiation, also curing by light/radiation may be applied.Alternatively or additionally, heat may be applied. Hence, especiallythe curable material is a thermally curable material. Therefore, in anembodiment the curable material comprises a thermally curable material,and the method further comprises subjecting at least part of the 3Dprinted object to heat (to cure the curable material). Hence, the 3Dobject, when under construction and/or when finished, may be cured, e.g.by heat.

As indicated above, the method includes 3D printing. Hence, the methodmay thus include using a 3D printer. Herein, the 3D printer mayespecially include a heating element (also) having the function to heatthe printed material and/or the curable material downstream from a 3Dprinter nozzle. This may be a heating of the printable material and/orthe curable material downstream from the nozzle but not yet deposited ona receiver item (or substrate) or on (other) printed material, and/orthis may be a heating of the printed material and/or the curablematerial. Especially, this heating may be a local heating, e.g. in aregion substantially directly below the printer nozzle. Hence, in anembodiment the 3D printer further comprises a heating unit configured toheat said printed material and/or the curable material.

The invention also provides a 3D printed object obtainable (orespecially obtained) with the method as described herein. Especially, inyet a further aspect the invention also provides a 3D printed objectcomprising a channel comprising cured material (i.e. a reinforcingstructure), wherein especially the cured material has a lower stiffnessthan the surrounding printed material. Further, as indicated above, thechannel may comprise an anchoring part selected from the groupconsisting of a bifurcation structure and a stitch structure. In aspecific embodiment, the 3D printed object comprises two or more layers,wherein the channel is configured within at least part of at a firstlayer and at least part of a second layer, and wherein a first anchoringpart is configured in the first layer and wherein a second anchoringpart is configured in the second layer. Of course, as also indicatedabove, the 3D printed object may also include a plurality of suchchannels.

The difference in stiffness indicates that the cured material in the 3Dprinted object is less stiff than the surrounding 3D printed material ofthe 3D printed object. Hence, the curable material and the printablematerial are selected to provide cured material and printed material,respectively, wherein the latter has a higher stiffness than the former.The stiffness may be tested with known material analysis techniques. Thestiffness of a material may be quantified in terms of Pascal or N/m²,and it may relate the stress in a material to the elastic strain (byHooke's law, which we can assume to be valid in the elastic regime of amaterial). Its value can be experimentally established from mechanicaltests such as a tensile test, bending test, a compression test, anindentation test, etc.

The materials may be tested on such characteristics making use ofstandard tests or analogies thereof. Note that herein especiallyrelative values are indicated. Examples of test which may be used, or ofwhich analogous tests based thereon may be used are e.g. (i) tensiletest: ASTM D638-10, ASTM D412-06a, ISO 37:2011, ISO 527; (ii) DMTA(dynamic mechanical thermal analysis): ASTM D4065-12, D5279-13, ASTME2254-13, ASTM E2425-11, ISO 4664-1:2011, ISO 4664-2:2006; (iii)compression/indentation test: ASTM D575-91 (2012), ASTM E2546-07, ASTMD2240-05 (2010). Note that the curable material may also be printed.Herein, however, the term “printed material” especially refers to thenon-curable material that is printed, especially with a 3D printer, inthe method for the production of the 3D printed object, although thecurable material may thus in some embodiments also be a “printedmaterial”. Hence, in an embodiment the surrounding printed material hasa first stiffness and the cured material has a second stiffness, whereinthe ratio of the second stiffness and the first stiffness is <0.8, suchas <0.5, like in the range of 0.001-0.05. Hence, the cured material ismore compliant. This ratio may apply for, for example, one or more ofbending (the ability to bend), compression (the ability to becompressed), and indentation (the ability to be indented), etc. (seealso above indicated tests). For instance, the cured material may betwice as bendable as the printed material, i.e. the stiffness is twiceas low (a ratio of 0.5). Alternatively or additionally, the ability tobe compressed may be higher for the cured material than for the printedmaterial, and thus the stiffness of the printed material is higher thanof the cured material, etc. Note that especially the printed materialthat is in physical contact with the cured material may be used tocompare. Hence, by introducing channels with more pliable material, the3D printed materials may be further kept together, like a kind ofconventional stitching.

In yet a further aspect, the invention also provides a 3D printercomprising a printer head comprising a first nozzle for printing a 3Dprintable material to a receiver item, the 3D printer further comprisinga second printer nozzle for providing a curable material, and whereinthe 3D printer further comprises a curing unit configure to cure thecurable material downstream from the second printer nozzle. The curingunit may include one or more of a heating unit and a radiation unit(such as for providing one or more of UV, VIS and IR radiation,especially at least UV radiation).

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIGS. 1a-1d schematically depict some basic aspects;

FIG. 2 very schematically show some stages and aspects of an embodiment;

FIGS. 3a-3h schematically depict some aspects of the invention;

FIG. 4 schematically depict an embodiment of a 3D printer (or AMprinter);

FIG. 5 show comparative results, with ND indicating the normalizeddisplacement and NRF indicating the normalized reaction force.

The schematic drawings are not necessarily on scale.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Additive manufacturing (AM) techniques based on powder sintering resultin objects composed of sintered grains where the neck between grains,indicated with reference 1320, has low fracture strength, see e.g. FIG.1a . AM based on layered processes (such as stereo lithography, sheetlamination) may result in objects composed of layers, indicated withreference 2320, where the interface between the layers represents apotential breakage line, see FIG. 1b . AM based on the deposition offilaments (Fused Deposition Modelling) result in objects composed offilaments, indicated with reference 320, where the interface between thefilaments represents a potential fracture line, see FIG. 1c . In fact,due to processing and resulting temperature differences, in-plane (IP)and out-of-plane (OP) adhesion properties will vary, thereby renderingthe out-of-plane properties inferior to the in-plane properties. AMbased on the gluing of grains or fibers (color-jetting, Mark Forgedfiber printer) result in parts composed of grains 1320 and/or fibers 320embedded in a matrix, indicated with reference 3320. The adherence ofthe matrix to the grains or the fibers might be weak, see also FIG. 1 d.

Possible solutions to improve e.g. in-plane and out-of-plane propertiesare for instance (i) reducing the effects of the heterogeneity of thestructure (e.g. local heating to improve the adhesion between layers,(ii) chemical or mechanical modification of the layers surfaces toimprove the adhesion, (iii) reducing the effect of the existingheterogeneity by a more continuous structure, (iv) removing or fillingthe voids, and (v) adding (vertical) stitches or other types ofreinforcing structures, which are especially herein described.

FIG. 2 very schematically show some stages and aspects of an embodiment.The method comprises (i) a 3D printing stage, indicated with referenceI, which may comprise 3D printing a 3D printable material 110 to providethe 3D printed object 100 of printed material 120, wherein the 3Dprinting stage further comprises forming during 3D printing a channel200 in the 3D printed object 100 under construction, wherein the methodfurther comprises (ii) a filling stage, indicated with reference II,comprising filling the channel 200 with a curable material 140 andcuring the curable material 140 to provide the channel 200 with curedmaterial 150. Optionally, the method may further comprises (iii) afinishing stage, indicated with reference III, subsequent to the fillingstage II, wherein the finishing stage comprises closing a channelopening 207, optionally but not necessarily by 3D printing. Note thatthe stages of filling the channels and curing the material maysubstantially be independent. Curing does not have to occur after everyfilling stage, it can occur after a certain number of filling stages ormaybe just once at the end—depending on the printed and curable materialproperties and the curing mechanism. Alternatively, if the ambienttemperature is high enough, no explicit curing action might benecessary, as over time the material will cure at this elevatedtemperature; i.e. automatically a curing stage may be included.Especially however, the printed material is subject to a temperatureover ambient temperature. As indicated above, curing may also be doneafter e.g. at least part of a finishing stage, e.g. a finishing stageincluding closing the channel 200.

FIGS. 3a-3h schematically depict some aspects of the invention. One orseveral stitches, or other types of reinforcing structures, can beintroduced per 3D object. The stitches or other types of reinforcingstructures, can have different shapes in three dimensions, provided bythe geometric design flexibility brought by AM, see e.g. FIG. 3a-3b ,wherein schematically some aspects of the herein described method aredepicted. FIG. 3a schematically depicts different types of channels 200filed with cured material. These channels promoting association ofdifferent parts of the body, like conventional stitches. As can bederived from FIG. 3a , the total volume of the channels 200 can berelatively low (compared to the total volume of the 3D printed object100). In FIG. 3a-3b , cavities are (were) introduced in the object 100during the printing process. FIG. 3c schematically depicts thatsubsequently the cavity is filled with a low-viscosity material (that iscurable), i.e. curable material 140.

The stitch or other types of reinforcing structure material can be e.g.silicone rubber, polysilazanes, resins, acrylates. To estimate therequired properties of the insert material, numerical simulations havebeen performed (see below). Due to the low volume fraction of the insertmaterial, the overall physical properties do not change significantly,though the strength of the 3D object may thus improve. The reinforcingstructures comprise one or more channels, and optionally one or moreanchoring parts (see below).

While filling the cavity, a pressure outlet 1207 may be needed, see alsoFIG. 3d . It is possible that the porosity in the object is enough tolet air out. If not, one can introduce an air outlet during the printingprocess. Of course, more than one pressure outlet 1207 may be applied.

In an embodiment, the inlet 207 may also be used as air outlet orpressure outlet 1207, which is especially during the filling stagepositioned on top of a part of the cavity so that the filling materialdoes not escape, as exemplified here (see FIGS. 3b-3e ). The air outletcan also be connected to vacuum in order to facilitate the filling ofthe cavity, allowing the use of higher viscosity material. The fillingstage can be executed while the 3D object is under reduced pressure.Once the cavity is filled, the material can be cured with heat, or light(if the part is transparent to the appropriate wavelength), or reactivegas (if the part is permeable to that gas).

In embodiments, see e.g. FIG. 3e , the top and bottom layer of the partare not maintained by the stitch, so they may have inferior mechanicalproperties compared to the rest of the part. These layers can be addedas sacrificial layer, indicated with reference 1120, on purpose, in away that allows easy removal. They can be removed by polishing ordissolved if they are made from another material than the part (e.g. thetop and bottom layer can be made of PVA that dissolves in water and is astandard material for support structures in Fused Deposition Modelling).This can be done during a finishing stage.

This method can also be used to improve the adherence between twomaterials and to “attach” two materials together that for instance mightnormally not adhere to each other at all, or normally have a too weakadherence, for an implementation in a product, as shown in FIG. 3f Here,the 3D printed object 100 comprises two or more layers 160, wherein thechannel 200 is configured within at least part of a first layer 160 aand at least part of a second layer 160 b. FIGS. 3e-3f e.g. show thatdifferent body parts are associated also via the reinforcing structure.

FIGS. 3g-3h further schematically depict some aspects of the invention.FIG. 3g for instance schematically depicts an embodiment of the channel200 comprises two or more channel parts 201 forming said channel 200,wherein each channel part 201 has a channel axis 202 (herein alsoindicated as “channel part axis”), and wherein two or more channel axes202 have a mutual angle α larger than 0° and smaller than 180°. Here,the channel parts are indicated with references 201 a-201 c and theirchannel (part) axes are indicated with references 202 a-202 c. Themutual angles are indicated with references α1-α3. Here, α1 is an acuteangle; the other angles α2 and α3 are by way of example obtuse (reflex).Of course, right angles may also be possible. FIG. 3h schematicallydepict some embodiments of possible anchoring parts 205, such asbifurcation structures 206 and stitch structures 207. Combinations mayalso be used (see e.g. FIG. 3a ). The anchoring parts, in combinationwith the channels may especially be used as reinforcing structures.

To prepare such stitches, or other types of reinforcing structures, the3D printer machine used for (added) manufacturing may need to have anadditional ink-jet or dispensing head that can be used to fill thecreated stitch holes with the appropriate stitching material. FIG. 4schematically depict an embodiment of a 3D printer that might e.g. beused for the AM method as described herein. This FIG. 4 shows a 3Dprinter 500 comprising a printer head 501 comprising a first nozzle 502for printing a 3D printable material 110 to a receiver item 550, the 3Dprinter 500 further comprising a second printer nozzle 1502 (forinstance from another printer head 1501) for providing a curablematerial 140, and wherein the 3D printer 500 further comprises a curingunit 1100 configure to cure the curable material 140 downstream from thesecond printer nozzle 1502. The curing unit 1100 may for instanceprovide heat, indicated with the reference q. By way of example, anembodiment of an FDM printer is schematically depicted.

Reference 500 indicates a 3D printer. Reference 530 indicates thefunctional unit configured to 3D print, especially FDM 3D printing; thisreference may also indicate the 3D printing stage unit. Here, only theprinter head for providing 3D printed material, such as a FDM 3D printerhead is schematically depicted. Reference 501 indicates the printerhead. The 3D printer of the present invention may especially include aplurality of printer heads, though other embodiments are also possible.Reference 502 indicates a printer nozzle. The 3D printer of the presentinvention may especially include a plurality of printer nozzles, thoughother embodiments are also possible. Reference 320 indicates a filamentof printable 3D printable material (such as indicated above). For thesake of clarity, not all features of the 3D printer have been depicted,only those that are of especial relevance for the present invention. The3D printer 500 is configured to generate a 3D item 10 by depositing on areceiver item 550 a plurality of filaments 320 wherein each filament 20comprises 3D printable material, such as having a melting point T_(m).The 3D printer 500 is configured to heat the filament material upstreamof the printer nozzle 502. This may e.g. be done with a devicecomprising one or more of an extrusion and/or heating function. Suchdevice is indicated with reference 573, and is arranged upstream fromthe printer nozzle 502 (i.e. in time before the filament material leavesthe printer nozzle 502). Reference 572 indicates a spool with material,especially in the form of a wire. The 3D printer 500 transforms thisinto a filament or fiber 320. Arranging filament by filament andfilament on filament, a 3D item 10 may be formed. The 3D printingtechnique used herein is however not limited to FDM (see also above).

To illustrate the invention, numerical simulations were performed with afinite element model. A representative geometry for a typical FDMfilament structure is chosen. The filaments are assumed to be ABS withstifnessE=2500 MPa, Poisson's ratio v=0.4, yield strength 25 MPa andhardening modulus H=125 MPa. The adhesion between the filaments isdescribed by so-called cohesive zone elements that define the separationbetween filaments by means of a traction-separation law in terms offracture toughness (G_(c)) and fracture strength (t_(max)). Thesenonlinear elements have been applied successfully to describe interfacefailure in microelectronic devices. To illustrate the effect ofprocessing (i.e. the difference in adhesion properties between in-planeand out-of-plane direction), the following adhesion properties arechosen: (a) vertical interfaces: G_(c)=8000 J/m², t_(max)=100 MPa; (b)horizontal interfaces: G_(c)=1000 J/m², t_(max)=35 MPa. The fracturetoughness of the vertical interfaces is based on the actual fracturetoughness of ABS. In order to load the in-plane and out-of-planeinterfaces equally, equi-biaxial strain is prescribed at the right andtop edges of the model, while symmetry conditions are applied at theleft and bottom edge. It appeared clearly that the level of out-of-planeadhesion results in a failing interface (damage). Due to the differencein adhesion properties, only a small amount of damage is initiating atthe in-plane interfaces. Clearly, these failures result in deterioratedmechanical properties of the printed structures. The resultingforce-displacement curves in horizontal and vertical direction exhibit aclear instability after which the interface failure localizes in thebottom row of interfaces instead of showing a more uniform interfacefailure throughout the specimen. To prevent these failures fromoccurring, a compliant and tough material is inserted between thefilaments. Due to the biaxial loading, a ‘+’-shape (cross-shape) of theinsert material is chosen for illustration purposes of the proposedmethod. In reality, the cavities between the filaments and the fillermaterial, will be filled as well. Clearly, this will improve theattachment of the filler material to the filaments due to the mechanicalinterlocking mechanism.

The deformed geometry, as a result of the equi-biaxial loading wasevaluated. It was found that the insert material with a lower stiffnessthan the surrounding material facilitates all deformations while theinterfaces are not critically loaded. To estimate the required adhesionto prevent the insert material from detaching from the filaments, energyrelease rate values have been calculated at several locations at theinterface between filler material and ABS. It turns out that therequired level of adhesion, for this specific case, varies between 45J/m² (for a very compliant material with E=1 MPa) up to 23 kJ/m² (for a‘stiff’ polymer material with E=2000 MPa). Typical polymer-polymerinterface adhesion values are in the order of several hundred J/m²(depending on the actual material combinations and surface treatments).As mentioned before, the filling of the cavities between the filamentsand the filler material during processing will alleviate thisrequirement due to the mechanical interlocking effect and increasingadhesive surface. Clearly, each specific case results in specificdemands on the insert material regarding compliance, toughness andinsert geometry. The latter can be easily taken into account in the 3Ddesign of the part. FIG. 5 shows the normalized reaction force asfunction of the normalized displacement for a structure withoutreinforcement structure (break at 0.5) and with reinforcement structure.

The term “substantially” herein, such as in “substantially consists”,will be understood by the person skilled in the art. The term“substantially” may also include embodiments with “entirely”,“completely”, “all”, etc. Hence, in embodiments the adjectivesubstantially may also be removed. Where applicable, the term“substantially” may also relate to 90% or higher, such as 95% or higher,especially 99% or higher, even more especially 99.5% or higher,including 100%. The term “comprise” includes also embodiments whereinthe term “comprises” means “consists of”. The term “and/or” especiallyrelates to one or more of the items mentioned before and after “and/or”.For instance, a phrase “item 1 and/or item 2” and similar phrases mayrelate to one or more of item 1 and item 2. The term “comprising” may inan embodiment refer to “consisting of” but may in another embodimentalso refer to “containing at least the defined species and optionallyone or more other species”.

Furthermore, the terms first, second, third and the like in thedescription and in the claims, are used for distinguishing betweensimilar elements and not necessarily for describing a sequential orchronological order. It is to be understood that the terms so used areinterchangeable under appropriate circumstances and that the embodimentsof the invention described herein are capable of operation in othersequences than described or illustrated herein.

The devices herein are amongst others described during operation. Aswill be clear to the person skilled in the art, the invention is notlimited to methods of operation or devices in operation.

It should be noted that the above-mentioned embodiments illustraterather than limit the invention, and that those skilled in the art willbe able to design many alternative embodiments without departing fromthe scope of the appended claims. In the claims, any reference signsplaced between parentheses shall not be construed as limiting the claim.Use of the verb “to comprise” and its conjugations does not exclude thepresence of elements or steps other than those stated in a claim. Thearticle “a” or “an” preceding an element does not exclude the presenceof a plurality of such elements. The invention may be implemented bymeans of hardware comprising several distinct elements, and by means ofa suitably programmed computer. In the device claim enumerating severalmeans, several of these means may be embodied by one and the same itemof hardware. The mere fact that certain measures are recited in mutuallydifferent dependent claims does not indicate that a combination of thesemeasures cannot be used to advantage.

The invention further applies to a device comprising one or more of thecharacterizing features described in the description and/or shown in theattached drawings. The invention further pertains to a method or processcomprising one or more of the characterizing features described in thedescription and/or shown in the attached drawings.

The various aspects discussed in this patent can be combined in order toprovide additional advantages. Furthermore, some of the features canform the basis for one or more divisional applications.

1. A 3D printed object comprising: a channel comprising cured material,wherein the channel comprises two or more channel parts, each channelpart having a channel axis, and two or more channel axes having a mutualangle greater than 0° and less than 180°, wherein the cured material hasa lower stiffness than the 3D printed material surrounding the curedmaterial, and wherein the channel further comprises an anchoring parthaving at least one of a bifurcation structure or a stitch structure. 2.The 3D printed object according to claim 1, wherein the 3D printedobject comprises two or more layers, and wherein the channel isconfigured within at least part of a first layer and at least part of asecond layer.
 3. The 3D printed object according to claim 2, wherein thechannel further comprises a first anchoring part and a second anchoringpart, the first anchoring part being configured in the first layer andthe second anchoring part being configured in the second layer.
 4. The3D printed object according to claim 1, wherein the cured materialcomprises a thermally cured material.
 5. The 3D printed object accordingto claim 1, wherein the cured material comprises a light cured material.6. The 3D printed object according to claim 1, wherein the 3D printedmaterial surrounding the cured material has a first stiffness, whereinthe cured material has a second stiffness, and wherein the ratio of thesecond stiffness and the first stiffness is less than 0.8.
 7. The 3Dprinted object according to claim 1, wherein the cured materialcomprises one or more of a polysiloxane, a polysilazane, a polyurethane,an epoxy, a polyamide, a polyimide, a polyester, and an acrylate.
 8. The3D printed object according to claim 1, wherein the 3D printed materialcomprises one or more of a polymeric material selected from the groupconsisting of ABS, polystyrene, and polycarbonate.
 9. The 3D printedobject according to claim 1, wherein the anchoring part has abifurcation structure having three or more splits.
 10. The 3D printedobject according to claim 1, wherein the anchoring part has abifurcation structure having a loop structure.
 11. The 3D printed objectaccording to claim 10, wherein the loop structure comprises a knot.